FERN GAZ. 19(2):55-62. 2012 55 DIPHASIASTRUM TRISTACHYUM (PURSH) HOLUB (LYCOPODIACEAE: LYCOPODIOPHYTA) – AN OVERLOOKED EXTINCT BRITISH NATIVE F. J. RUMSEY Angela Marmont Centre for UK Biodiversity, Natural History Museum, Cromwell Road, London, SW7 5BD, UK. (Email: [email protected]) Key Words: Clubmoss, Diphasiastrum alpinum, Diphasiastrum × issleri, Hampshire, herbarium ABSTRACT During a study of herbarium material previously ascribed to Diphasiastrum × issleri (Rouy) Holub specimens belonging to the continental European and North American D. tristachyum (Pursh) Holub, a taxon not hitherto definitely recorded from the British Isles, were found. While its identity was correctly suggested by Babington when found, it was initially reported as D. complanatum before later being placed in D. alpinum and subsequently formed the first accepted British record for D. × issleri. The status of this material is discussed and it is considered to be native to the British Isles. Although not re-found since its discovery in 1866, some suitable habitat remains; given the decline in all lycopods in lowland England it is, however, best considered extinct (EX) and unlikely to re-appear. INTRODUCTION The clubmosses of the genus Diphasiastrum have long posed taxonomic and nomenclatural conundrums to European botanists, not least through the widespread occurrence of intermediate fertile hybrids, the rank of which has been variously interpreted (cf. Jermy, 1993; Kukkonen, 2000). Given their fertility one might expect a near morphological continuum between taxa and yet there remains little solid evidence for the backcrossing of the fertile intermediate F1 hybrids, D. × issleri (Rouy) Holub, D. × zeilleri (Rouy) Holub and D. × oellgaardii Stoor, Boudrie, Jérôme, K.Horn & Bennert with their parental taxa D. alpinum (L.) Holub, D. complanatum (L.) Holub and D. tristachyum (Pursh) Holub (Bennert et al., 2011). The cause(s) of this apparent failure is still unclear. However, plants that are difficult to place morphologically are regularly encountered, particularly it would seem in the British Isles, and introgression has been invoked to account for these (Jermy, 1989). Within Britain there has long been recognition of the existence of plants which could not be ascribed with confidence to the widespread montane D. alpinum, but determining their true identity and the characters by which they might be diagnosed has been less satisfactorily dealt with. Earlier authors (eg. Druce, 1882) regarded some of these plants as states of D. complanatum but it was only comparatively recently that some clarity was brought to the situation by the recognition that these represented hybrids between that taxon and D. alpinum (D. × issleri), the hybrid apparently being present in the absence of the D. complanatum parent (Jermy, 1989). The taxonomic treatment currently adopted for 56 FERN GAZ. 19(2):55-62. 2012 this (see Stace, 2010), as a subspecies under D. complanatum, is regrettable, being both misleading and confusing. Jermy (1989) considered that there may be two ecotypes of this hybrid present, one lowland in England, the other more montane and Scottish, which accorded more closely with typical continental material. He speculated that a Malvern (v.c.37) plant, now sadly lost, was the result of introgression with D. alpinum, although Kukkonen (2000) considered it to be atypical D. alpinum. Concerns over the identity of some of the lowland examples regarded as this hybrid continue. Difficulties in identification have generally been ascribed to morphological plasticity as a consequence of environmental variables (shade, moisture, juvenility, etc.) and yet recent molecular and cytological studies (Bennert et al., 2011) have revealed that some anomalous morphologically intermediate individuals are triploids, almost certainly derived from the mating of diploid hybrid gametophytes from unreduced gametes (diplospores) with normal haploid gametophytes. It was while attempting to resolve these issues in a British context that a more thorough review of the historical herbarium collections at BM and elsewhere was made and the mis-identification of some specimens became apparent. Most significant was the discovery of unequivocal material of D. tristachyum, a plant not hitherto considered to be present in the British Isles. DIPHASIASTRUM TRISTACHYUM IN BRITAIN A single specimen of Diphasiastrum tristachyum and a photograph of a more fragmentary specimen from the same gathering held at CGE were found amongst specimens filed as D. × issleri at BM (Figures 1, 2) The material, although sterile, clearly shows the distinctive foliage and habit (see below) of this species. The D. tristachyum material is actually the first and earliest specimen considered by Jermy (1989) to be Diphasiastrum × issleri. It was initially published as Lycopodium complanatum L. in an editorial note in the Gardener’s Chronicle for 11 August 1886 (Anon, 1866). This reported a communication from John Lloyd claiming “it was found by a woman named Sarah Young, while occupied in cutting Heath for broom-making”, at Lower Waggoner’s Wells, Bramshott, N. Hants. v.c.12 (SU 85*, 34*). Initially this identification was doubted, Babington (1867) saying “ I have not seen it, and doubt its being correctly named” but specimens gathered by Lloyd the following year (Lloyd, 1867a), sent to the editor of the Gardeners’ Chronicle as L. alpinum L., were referred to Babington who “inclines to place it under L. chamaecyparissus R. Br., a plant of the Rhine valley” (the differences between European material originally described as L. chamaecyparissus and the North American L. tristachyum Pursh are slight and all recent authors have treated them in synonymy (eg. Wilce,1965)). The editorial concluded “There are some doubts as to the true name of this Lycopodium, arising from the fact that the specimens sent are the barren stems only... It may be L. alpinum, but the subterranean rhizome is unlike that species”. Lloyd (1867a) had made particular reference to the buried rhizome, which is indeed a crucial diagnostic character for tristachyum and its hybrids, but later doubted the significance of this, having apparently found similar subterranean stems in Welsh L. alpinum (Lloyd, 1867b), an observation which may well have helped deliver his find into obscurity! Considerable confusion continued to surround the identity of this plant. Watson (1870) recorded the Bramshott plant as L. complanatum but by the time the 2nd edition of his Topographical Botany (Watson, 1883) was published he lists “12”, i.e. the record from North Hants. as requiring further investigation, while still accepting RUMSEY: DIPHASIASTRUM TRISTACHYUM 57 records of complanatum from v.c 33, 55, 90 and 102 . Babington also clearly changed his mind as to the identity of the Hants. plants over time. Initially acknowledging but not fully accepting complanatum in the 6th (1867) and 7th editions (1874) of his Manual, by the 8th edition (1881) he only lists alpinum as British but, shortly afterwards in an Addendum (Babington, 1883), he finally concedes the existence of complanatum, listing it as in Hants., Gloucs., Worcs., Ross and Skye. The existence of an earlier British specimen of D. tristachyum has, however, long been known, although it has generally been dismissed as resulting from an error in mounting and is not mentioned in recent floras (Stace, 2010) or guides (Page, 1997). Druce (1916) was the first to point out the existence of this unequivocal D. tristachyum (as L. chamaecyparissus) on a herbarium sheet in Herb. C. Bailey (MANCH) – see Figure 3. The shoot is not specifically numbered and thus its affiliation is open to doubt. IDENTIFICATION When fertile, Diphasiastrum tristachyum is easily recognised by its long pedunculate (4.5 -15cm) fertile shoot usually bearing four or more stalked strobili. The fertile shoots of D. alpinum bear 1-2 sessile strobili; those of D. × issleri also have fewer strobili, which are only shortly (<4cm) pedunculate. The sterile shoots of D. tristachyum are easily distinguished; the 2-3 times bifurcating branches are tightly clustered, fastigiate and all of a similar ultimate length and the narrow glaucous upswept foliage has a very neat appearance. The dorsal leaves are almost as broad as the lateral, not significantly narrower as in D. × issleri and to a lesser extent D. alpinum, and are more similar in shape to the ventral than in either of these. The main axis is subterranean, deeply buried and not just superficial in the litter layer as are those of the other British taxa. Diphasiastrum tristachyum is most easily confused with D. × zeilleri, its hybrid with D. complanatum. This differs in its more differentiated lateral leaves, broader (>2 vs. <1.5mm) branchlets and usually greener foliage. ASSESSING THE STATUS OF DIPHASIASTRUM TRISTACHYUM IN THE BRITISH ISLES Although doubt must surround the true provenance of the material reputedly collected on Ingleborough (v.c. 64), no such doubt attaches to the material from Bramshott. Clubmosses have never been successfully cultivated in this country and would not have been deliberately or unwittingly introduced in this area. The species can therefore either be considered as a chance colonist that failed to persist, or as a rare native that may once have been more widespread. The native range of D. tristachyum is broad, extending through Eastern North America, parts of Asia, the Caucasus and through much of central Europe extending northwards into the Scandinavian mountains (although not as far as its congeners), southwards to the northern Appenines of Italy and now reaching its western limits in Belgium and the Massif central in France. Like many clubmosses it would, however, seem to have been in regression for some time. Habitat destruction, changes in management and land use, even eutrophication and climate change are combining to contribute to this decline, this being particularly marked in the more westerly and lowland areas of this species range. In the mid 19th century the species occurred near Beauvais in the Paris region (Prelli, 2001) and there seems little reason from a phytogeographical perspective why it should not have occurred naturally on the heaths 58 FERN GAZ.